If thou canst answer me - The meaning of this verse is this: "The controversy between you and me, if you choose to reply, shall be conducted in the most equitable manner, and on the most equal terms. I will not attempt, as your three friends have done, to overwhelm you with reproaches; nor will I attempt to overawe you as God would do, so that you could not reply. I am a man like yourself, and desire that if anything can be said against what I have to advance, it should be offered with the utmost fairness and freedom."
Stand up - That is, "maintain your position, unless you are convinced by my arguments. I wish to carry nothing by mere authority or power."

If thou canst answer me,.... That is, when he had done speaking, after he had heard him out; if he thought he could make a reply to him, he gave him full liberty so to do, and tacitly suggests that he should give him an attentive and candid hearing, as he had requested of him:
set thy words in order before me; put them into the best form and order thou canst for thy self-defence, and level them at me; set them, as it were, in battle array against me; give them all the poignancy, strength, and three thou art capable of:
stand up; not out of veneration to him, but to denote freedom and boldness in himself; a presentation of himself with boldness, and standing and keeping his ground: the expressions are military; Mr. Broughton renders it, "stand to it".

Images from a court of justice.
stand up--alluding to Job's words (Job 30:20).